For measuring a propagation delay of an optical signal between two optical devices connected by two separate links, one for each transmission direction, a method is known for measuring a forward and return propagation delay. A measurement signal is sent by an optical device initiating the measurement to the neighboring optical device via the first optical link. The latter responds to the measurement signal to the initiating optical device via the second optical link.
The Precision Time Protocol (PTP), as specified in the standard IEEE 1588-2008, enables a communication network to synchronize clocks between devices. Several mechanisms are possible in this standard. One of them comprises a mechanism for calculating optical delay, called ‘Peer Delay Link Measurement’. In this mechanism, an initiating device sends a ‘Peer_Delay_Request’ message to a remote device via a first optical link. The latter responds using a response message ‘Peer_Delay_Response’ optionally accompanied by another response message ‘Peer_Delay_Response_Follow_Up’, one of these two response messages including in particular a measurement of an internal electronic processing delay to the remote device. This internal processing delay may vary, for example, according to the load factor of the remote device. This or these response messages are routed via a second optical link, separate from the first optical link. This method therefore enables the internal processing delay in the remote device to be precisely obtained for a given measurement.
A forward and return propagation delay is then obtained by taking away the internal processing delay as it was received from the remote device, from the delay measured by the initiating device. The propagation delay in one direction is then estimated by dividing the forward and return propagation delay into two equal parts. However, this method does not take into account an asymmetry of the optical links connecting the initiating and remote devices.
Thus, in an optical transmission network, when the respective lengths of the links of the two directions are not identical, the value of the estimated propagation delay is affected by an error that is hard to quantify.
In some cases, when there is a difference in length of the links, e.g. optical fibers, the asymmetry can be significant. In other cases, although the difference in length of the fibers in itself is quite small, particularly when they belong to the same cable, asymmetry may arise from the distribution of fibers on the devices themselves or from repairs or splices made during the service life of the cable.
Many applications require a knowledge of this propagation delay of an optical signal from one device to another. Methods of resource reservation or methods of synchronization of optical devices may be cited by way of example.
For these different applications, the precision required for measuring a propagation delay of an optical signal is of the order of a few nanoseconds. It cannot be obtained using existing techniques, in particular due to the error affecting the measurement, especially in the case of asymmetric links.